After dealing with the question of defining what the truth is for this situation, and questions of self-deception and so on, the next big question is whether you’re speaking of suddenly imposing such a restriction on existing society, or wondering what a society would be like that developed under this restriction in the first place.
A society that developed under such a restriction would likely be nigh-unrecognizable to us. All of our social interaction and culture is touched upon by falsehoods great and small, and it developed that way. Every interaction we now take for granted would have to be examined, even if it doesn’t contain lying or falsehood, to see if the practice originated in something that was caused by the possibility of falsehood.
Consider a common greeting custom like shaking hands. As far as I’m aware, we don’t know for certain why this tradition originated, but I’ve read it’s suspected to be meant as a demonstration that the individuals have no weapons in their right hands. Would such a custom have ever developed in a world where if you declare you come in peace, it is known to be true? Unlikely.
I’ve thought about this a few times myself in the past, but never really been able to figure out what a world like that would look like, since it’s so vastly alien in every way, down to the very basics of culture, that we can’t assume anything about human development happened in even a vaguely similar way (obviously all of history would be different, at least).
On the other hand, if such a rule were suddenly imposed on the world today, I imagine it would lead to a complete collapse of the social order. Society and humanity would probably survive in the long term, but it would be a very, very rough patch. Millions would be unwilling to go out and do anything, speak to anyone, and so on. Many more would simply refuse to speak at all, saying nothing most of the time. Entire swaths of the economy would collapse overnight, international relations would instantly grind to a halt.
International relations? Hell, it’d be lucky if governments continued to function at all. Optimistically, all crucial functions would still be performed, but anything beyond that, it’d be hard to say. I wonder if we’d have actual angry mobs demanding answers now that politicians are forced to speak the truth?
I think I am curious and will have to read some of the books mentioned above. The movie I have seen and found to be okay, but not really something to look at in a serious consideration of this topic. It was a comedy and played for laughs, not a serious exploration of what a world without the ability to lie would look like.